Discovery of very high energy γ-ray emission from the BL Lacertae object H 2356-309 with the HESS Cherenkov telescopes

Abstract

The extreme synchrotron BL Lac object H 2356-309, located at a redshift of z = 0.165, was observed from June to December 2004 with a total exposure of ≈40 h live-time with the HESS (High Energy Stereoscopic System) array of atmospheric-Cherenkov telescopes (ACTs). Analysis of this data set yields, for the first time, a strong excess of 453 γ-rays (10 standard deviations above background) from H 2356-309, corresponding to an observed integral flux above 200 GeV of I(>200 GeV) = (4.1 ± 0.5) × 10-12 cm-2 s-1 (statistical error only). The differential energy spectrum of the source between 200 GeV and 1.3 TeV is well-described by a power law with a normalisation (at 1 TeV) of N0 = (4.1 ± 0.5) × 10-13 cm-2 s-1 TeV-1 and a photon index of Γ = 3.09 ± 0.24stat ± 0.10sys. H 2356-309 is one of the most distant BL Lac objects detected at very-high-energy γ-rays so far. Results from simultaneous observations from ROTSE-III (optical), RXTE (X-rays) and NRT (radio) are also included and used together with the HESS data to constrain a single-zone homogeneous synchrotron self-Compton (SSC) model. This model provides an adequate fit to the HESS data when using a reasonable set of model parameters.

Astronomy & Astrophysics 455, 461-466 (2006)
ArXiv pre-print

Auxiliary informations

H.E.S.S. Spectrum (Figure 2, Table 4)

	Energy	Flux			Flux Error
	[TeV]	[1/(cm2 s TeV)]		[1/(cm2 s TeV)]
  	0.223	2.06e-11		7.13e-12
  	0.270	2.18e-11		4.36e-12
  	0.329	1.04e-11		2.46e-12
  	0.403	5.77e-12		1.48e-12
  	0.494	2.12e-12		8.51e-13
  	0.604	1.26e-12		5.45e-13
  	0.742	5.50e-13		3.29e-13
  	0.912	5.77e-13		2.31e-13
  	1.113	<4.06e-13		--

Remarks: Statistical errors only. Systematic errors on energy scale are 20%. See paper for details.

Spectral energy distribution data (Figure 4)

experiment 		log(nu [Hz])		log(nuFnu [erg/cm2s])	flux error

Nancay Radio Telescope	9.40002         	-14.9979                0.0791812

ROTSE-III (lower)	14.6321 		-12.7427		--
          (upper)	14.6321 		-11.7696		--

RXTE			17.8326 		-11.0888                0.0351585
			17.8922 		-11.1968                0.0391323
			17.9447 		-11.2547                0.0430312
			18.0132 		-11.1891                0.0290805
			18.0904 		-11.2401                0.0383379
			18.1561 		-11.2215                0.0441935
			18.2391 		-11.346                 0.0515208
			18.3500 		-11.4396                0.0768071
			18.4842 		-11.3517                0.103189

Remarks: Statistical errors only. See paper text for details.

Table of Contents